Exhaust valve for an internal combustion engine

ABSTRACT

An exhaust valve for an internal combustion engine including a movable spindle with a valve disc of a nickel-based alloy which also constitutes an annular seat area at the upper surface of the valve disc. The seat area abuts a corresponding seat area on a stationary valve member in the closed position of the valve. At manufacturing, the seat area of the valve disc is subjected to a thermo-mechanical deformation process at a temperature lower than or around the recrystallization temperature of the alloy. The seat area on the upper surface of the valve disc has been given dent mark preventing properties in the form of a yield strength of at least 1000 MPa at a temperature of approximately 20° C. by means of the thermo-mechanical deformation process and possibly a yield strength increasing heat treatment.

1. CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from Danish patentapplication No. 641/96 filed Jun. 7, 1996 and from International patentApplication No. PCT/DK97/00245 filed Jun. 3, 1997.

2. BACKGROUND OF INVENTION

a. Field of Invention

The present invention relates to an exhaust valve for an internalcombustion engine. In particular it relates to a two-stroke crossheadengine, comprising a movable spindle with a valve disc of a nickel-basedalloy which also constitutes an annular seat area at the upper surfaceof the valve disc, which seat area abuts a corresponding seat area on astationary valve member in the closed position of the valve. The seatarea of the valve disc has been subjected at its manufacture to athermo-mechanical deformation process at which the material is at leastpartially cold-worked.

b. Description of Related Art

The development of exhaust valves for internal combustion engines hasaimed for many years at extending the life and reliability of thevalves. This has been done so far by manufacturing the valve spindleswith a hot-corrosion-resistant material on the lower disc surface and ahard material in the seat area.

The seat area is particularly crucial for the reliability of the exhaustvalve, as the valve has to close tightly to function correctly. It iswell-known that the ability of the seat area to close tightly can bereduced by corrosion in a local area by a so-called burn through, whereacross the annular sealing surface a channel-shaped gutter emerges,through which hot gas flows when the valve is closed. Under unfortunatecircumstances, this failure condition can arise and develop into arejectable valve during less than 80 hours' operation, which means thatoften it is not possible to discover the beginning failure at the usualoverhaul. Therefore, a burn through in the valve seat may causeunplanned shut-downs. If the engine is a propulsion engine in a ship,the failure may arise during a single voyage between two ports, whichmay cause problems during the voyage and unintended expensive waitingtime in port.

With a view to preventing burn throughs in the valve seat many differentvalve seat materials with ever increasing hardness have been developedover the years to make the seat wear-resistant by means of the hardnessand reduce the formation of dent marks. The dent marks are a conditionfor development of a burn through as the dents may create a small leakthrough which hot gas flows. The hot gas can heat the material aroundthe leak to a level of temperature where the gas with the aggressivecomponents has a corrosive effect on the seat material so that the leakrapidly grows larger and the leakage flow of hot gas increases, whichescalates the erosion. In addition to the hardness, seat materials havealso developed towards a higher hot corrosion resistance to delayerosion after the occurrence of a small leak.

An exhaust valve of the above type and manufactured from the materialNIMONIC 80A is described in the article ‘Herstellung von Ventilspindelnaus einer Nickelbasislegierung für Schiffsdieselmotoren’, Berg- undHüttenmännische Monatshefte, volume 130, September 1985, No. 9. Thethermo-mechanical forging is controlled so that a high hardness isachieved in the seat area. In consideration of the mechanical propertiesof the exhaust valve, such as fatigue resistance, etc., the articleprescribes that the NIMONIC 80A valve has a yield strength of at least800 MPa.

EP-A-0 280 467 describes an exhaust valve made of NIMONIC 80Amanufactured from a base body forged into the desired shape aftersolution annealing. The seat area is thus cold-worked for provision ofhigh hardness. Subsequently the valve can be precipitation-hardened.

The book ‘Diesel engine combustion chamber materials for heavy fueloperation’ published in 1990 by The Institute of Marine Engineers,London, collects the experience gained for exhaust valve materials in anumber of articles and provides recommendations as to how to designvalves to achieve long life. Concerning valve seats the articlesunanimously direct that the seat material has to have a high hardnessand be of a material with a high resistance against hot corrosion. Anumber of different preferred materials for exhaust valves are describedin Paper 7 of the book ‘The physical and mechanical properties of valvealloys and their use in component evaluation analyses’, including in itsanalysis of the mechanical properties of the materials a comparativetable of the yield strength of the materials, seen to be below about 820MPa.

3. BRIEF SUMMARY OF INVENTION

It is desirable to prolong the life of the exhaust valve andparticularly to reduce or avoid unpredictable and rapid development ofburn throughs in the seat area of the valve. The Applicant has carriedout tests with dent mark formation in seat materials and contrary to theestablished knowledge has established quite unexpectedly that thehardness of the seat material does not have any great influence onwhether the dent marks emerge.

The object of the present invention is to provide seat materials thatanticipate the mechanism leading to formation of dent marks, whereby thebasic condition for occurrence of burn throughs is weakened oreliminated.

In view of this the exhaust valve according to the invention ischaracterized in that the valve disk is made of a nickel-based alloywhich can achieve a yield strength of at least 1000 MPa, and that theseat area at the upper surface of the valve disc has been given dentmark preventing properties in the form of a yield strength (R_(p0.2)) ofat least 1000 MPa at a temperature of approximately 20° C. by means ofthe thermo-mechanical deformation process and possibly a yield strengthincreasing heat treatment.

Dent marks are formed by particulate combustion residues, such as cokeparticles, which flow from the combustion chamber up through the valveand into the exhaust system while the exhaust valve is open. When thevalve closes, the particles may get caught between the closing sealingsurfaces on the valve seats.

From studies of numerous dent marks on valve spindles in operation ithas been observed that new dent marks very rarely reach the upperclosing rim, viz., the circumferential line at which the upper end ofthe stationary valve seat is brought into contact with the movableconical valve seat. In practice, the dents end about 0.5 mm away fromthe closing rim, which is without any immediate explanation, as aparticle can also be expected to be caught in this area.

It has now been realized that the absence of dents immediately up to theclosing rim is due to the fact that coke particles and other, even veryhard particles are crushed to powder before the valve is completelyclosed. Part of the powder is blown away simultaneously with thecrushing of the particles because the gas from the combustion chamberflows out through the gap between the closing sealing surfaces atapproximately sonic velocity. The high gas velocity blows the powdernear the closing rim away, and the absence of dents out to the rim showsthat just about all particles getting caught between the sealingsurfaces are pulverized. Even very thick particles are reduced inthickness by the crushing and blowing away of powder, and in practicethe subsided piles of powder capable of forming the dent marks thereforehave a highest thickness of 0.5 mm and a normal maximum thickness of0.3-0.4 mm.

Especially within the most recent engine development where the maximumpressure may be 195 bar, the load on the lower surface of the disc maycorrespond to up to 400 tons. When the exhaust valve is closed and thepressure in the combustion chamber rises to the maximum pressure, thesealing surfaces are pressed completely together around an enclosedpowder pile. This cannot be prevented, no matter how hard the seats aremade.

When combustion of the fuel commences and the pressure in the cylinderand thus the load on the valve disc increase, the enclosed powder pilestarts wandering into the two sealing surfaces and at the same time theseat materials are elastically deformed. During this elastic deformationthe surface pressure between the powder pile and the sealing surfacesrises, which usually makes the powder pile deform into a larger area. Ifthe powder pile is sufficiently thick, the elastic deformation continuesuntil the pressure in the contact area of the powder pile reaches theyield strength of the seat material of the lowest yield strength,whereupon this seat material is plastically deformed and formation ofthe dent mark commences. The plastic deformation may result in anincrease of the yield strength owing to deformation hardening. If thetwo seat materials in the local area around the powder pile thus achieveuniform yield strengths, the powder pile starts plastically deformingthe other seat material as well.

If the formation of dent marks is to be countered, this, as mentionedabove, cannot be done by making seat materials harder, instead they haveto be made resilient, which is obtained by manufacturing the seat areaswith a high yield strength. The higher yield strength provides a doubleeffect. Firstly, the seat material with the higher yield strength may beexposed to a higher elastic strain and thus absorb a thicker powder pilebefore plastic deformation occurs.

The second essential effect is associated with the surface nature of thesealing surfaces in the areas facing the powder pile. The dent profileformed by the elastic deformation is even and smooth and promotes thedistribution of the powder pile to a larger diameter, which partlyreduces the thickness of the powder pile, partly reduces stresses in thecontact area following from the greater contact area. At the transitionfrom elastic deformation to plastic deformation a deeper and moreirregular dent profile is rapidly created which will unsuitably anchorthe powder pile and thus have a preventive effect on a furtheradvantageous enlargement of the diameter of the pile.

Tests have shown that in an exhaust valve a powder pile of a thicknessof about 0.14 mm can be absorbed between two seat areas of materialswith a lower limit for the yield strength of 1000 MPa without anyplastic deformation of the sealing surfaces. A large proportion of theparticles caught between the seat surfaces will be crushed to athickness of about 0.15 mm. The exhaust valve according to the inventionprevents a noticeable proportion of the particles from forming dentmarks because the seat surface merely springs back to its original shapewhen the valve opens, and at the same time the remains of the crushedparticle are blown away from the seat surfaces.

In consideration of an increase of the elastic properties of the seatarea, it is preferred that the seat area material has a yield strengthof at least 1100 MPa, preferably at least 1200 MPa. Young's modulus forthe current seat material is substantially unchanged at increasing yieldstrengths, which gives an approximately linear correlation between yieldstrength and the highest elastic strain. It appears from the abovecomments that a seat material with a yield strength of 2500 MPa or morewould be ideal because it could absorb the powder piles of the normallymost frequently occurring pile thickness purely by elastic deformation.However, at present suitable materials with such a high yield strengthare not at hand. It will appear from the below description that some ofthe seat materials available today can be manufactured in a manner thatraises the yield strength to at least 1100 MPa. All other things beingequal, this 10% increase in yield strength will result in at least a 10%reduction of the depth of any dent marks. For most types of particles,the suitable limit of 1200 MPa is sufficiently high to provide anoticeable reduction of the pile thickness and consequently may resultin a reduction of the dent mark depths of up to 30%, but at the sametime the number of possible materials is narrowed down. This alsoapplies to seat materials with a yield strength of at least 1300 MPa.

In an especially preferred embodiment the seat area material has a yieldstrength of at least 1400 MPa. This yield strength is almost double theyield strength of the seat materials used at present, and based on thepresent understanding of the mechanism of dent mark formation thematerial with this high yield strength is presumed to largely eliminateproblems with seat area burn throughs. The depth of the few dent marksthat can be formed in this seat material will be too small for leakagegas to flow through the dent mark in sufficiently large quantities forthe seat material to be heated to a temperature where hot corrosionbecomes effective.

In one embodiment the seat areas on the stationary member and the valvedisc, respectively, have substantially the same yield strength at theoperating temperatures of the seat areas. The largely uniform yieldstrengths of the two seat materials result in approximately the samemanner of deformation of both sealing surfaces when the powder pile ispressed into the surfaces, which reduces the resulting plasticdeformation in each of the surfaces. The stationary seat area is colderthan the seat area on the spindle, which means that the spindle seatmaterial should have the higher yield strength at about 20° C. in viewof the fact that the yield strength for many materials drops atincreasing temperatures. This embodiment is especially advantageous ifthe stationary seat area is made of a hot-corrosion-resistant material.

If the stationary seat area is of hardened steel or cast iron, the seatarea on the stationary member preferably has a substantially higheryield strength than the seat area on the valve disc at the operatingtemperatures of the seat areas. With this design any dent marks will beformed on the valve spindle. This provides two advantages. Firstly theseat area on the spindle is normally made of a hot-corrosion-resistantmaterial so that any dent mark will find it more difficult to developinto a burn through than if the dent was located on the stationarymember. Secondly the spindle rotates so that at each valve closure thedent will be located at a new position on the stationary sealingsurface, the heat influence thus being distributed on the stationaryseat area.

The following materials are applicable according to the invention asvalve disc and seat materials. It should be noted that NIMONIC is aproprietary trademark of INCO Alloys.

Preferably the whole body or at least the whole valve disc is made of aNIMONIC alloy. Of these it is well-known to use NIMONIC 80, NIMONIC 80Aor NIMONIC 81, which have provided good operational experiences asregards wearing qualities and corrosion resistance in the corrosiveenvironment present in the combustion chamber of a large diesel engine.Also applicable is NIMONIC Alloy 105 which after casting andconventional forging of the base body has a yield strength of about 800MPa which has been brought to more than 1000 MPa after approximately 15%cold-working. Also NIMONIC PK50 is applicable and can be cold-worked andprecipitation-hardened to a yield strength of approximately 1100 MPa.With the conventional NIMONIC alloys and a degree of deformation of 70%in the seat area it is possible to achieve a yield strength ofapproximately 1400 MPa. It is also possible to increase the yieldstrength further through a precipitation-hardening heat treatment.

The choice of manufacturing process can be influenced by the size of theexhaust valve, as a cold-working of many per cent may require strongtools when the valve disc is large, for example, with an externaldiameter ranging from 130 mm to 500 mm.

The present invention also relates to the use of a nickel-basedchromium-containing alloy with a yield strength of at least 1000 MPa atapproximately 20° C. as a dent mark limiting or preventive material inan annular seat area at the upper surface of a movable valve disc in anexhaust valve for an internal combustion engine, particularly atwo-stroke crosshead engine, the seat area abutting a corresponding seatarea on a stationary valve member when the valve is closed. The specialadvantages of using such a dent mark limiting material appear from theabove description.

4. BRIEF DESCRIPTION OF THE DRAWINGS

Examples of embodiments of the invention will now be described below infurther detail with reference to the highly schematic drawing, in which

FIG. 1 is a longitudinal sectional view through an exhaust valveaccording to the invention,

FIG. 2 is a segmental view of the two seat areas with a typical dentmark sketched in,

FIGS. 3-6 are segmental views of the two seat areas illustrating theparticle crushing and the introductory steps in the dent mark formation,

FIGS. 7 and 8 are enlarged segmental views of the dent mark formation,and

FIG. 9 is a corresponding view of the surfaces immediately afterreopening of the valve.

5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an exhaust valve generally designated 1 for a largetwo-stroke internal combustion engine, which may have cylinder diametersranging from 250 to 1000 mm. The stationary valve member 2 of theexhaust valve, also called the bottom piece, is mounted in a cylindercover, not shown. The exhaust valve has a movable spindle 3 supportingat its lower end a valve disc 4 and, in a well-known manner, beingconnected at its upper end with a hydraulic actuator for opening of thevalve and a pneumatic return spring returning the spindle to its closedposition. FIG. 1 shows the valve in a partially open position.

If a higher corrosion resistance than achievable with the base materialis desired, the lower surface of the valve disc may be provided with alayer of hot-corrosion-resistant material 5. An annular seat area 6 onthe upper surface of the valve disc is at a distance from the outer rimof the disc and has a conical sealing surface 7. Although the seat areain the Figure has a different numerical designation than the disc, itshould be understood that both parts are made of the same alloy. Thevalve disc for the large two-stroke crosshead engine can have anexternal diameter in the interval from 120 to 500 mm depending on thecylinder bore.

The stationary valve member is also provided with a slightly projectingseat area 8 forming an annular conical sealing surface 9 which abuts thesealing surface 7 in the closed position of the valve. As the valve discchanges shape during heating to the operating temperature, the seat areais designed so that the two sealing surfaces are parallel at theoperating temperature of the valve, which means that on a cold valvedisc the sealing surface 7 only abuts the sealing surface 9 at thelatter's upper rim 10 located farthest away from the combustion chamber.

FIG. 2 illustrates a typical dent mark 11 ending approximately 0.5 mmaway from the closing rim on the sealing surface 7, viz., the circulararc where the upper rim 10 hits the sealing surface 7 as indicated bythe vertical dotted line.

FIG. 3 illustrates a hard particle 12 which is caught between the twosealing surfaces 7, 9 immediately before the valve closes completely. Atthe continued closing motion, the particle is crushed into powder, ofwhich a considerable part is entrained by the gas flowing up between theseats at sonic velocity as shown by the arrow A in FIG. 4. Part of thepowder from the crushed particle will be locked between the sealingsurfaces 7, 9 because the particles nearest the surfaces are retained byfrictional forces, and the particles in the inter-space are locked byshear forces in the powder. Thus, opposite conical powder piles areformed facing tip to tip. The assumption prevailing so far to the effectthat a solid particle is caught between the seat surfaces is thus notcorrect. Instead a reduction of the amount of material caught betweenthe seats occurs because part of the powder blows away.

During the continued closing motion, the conical powder accumulationscollapse and are spread in the plane of the surfaces to a lens-shapedpowder body or a powder pile, as illustrated in FIG. 5. This lens-shapedpowder body has proved to have a maximum thickness of 0.5 mm and anormal thickness for the largest accumulations of between 0.3 and 0.4mm.

FIG. 6 illustrates the situation when the valve is closed, but beforethe pressure in the combustion chamber rises as a consequence of thecombustion of the fuel. The pneumatic return spring is not in itselfstrong enough to pull the sealing surface 7 completely tight against thesealing surface 9 in the area around the powder body.

When the pressure in the combustion chamber rises after ignition of thefuel, the upward force on the lower disc surface rises strongly, and thesealing surfaces are pressed closer against each other. At the same timethe powder body starts deforming the sealing surfaces elastically. Ifthe powder body is sufficiently thick and the yield strength of thematerial is not sufficiently high, the elastic deformation will turninto plastic deformation making the dent permanent.

FIG. 7 illustrates a situation where the stationary seat area 8 has thehighest yield strength, and where the seat area 6 on the disc isdeformed elastically to just below its yield limit. At the continuedcompression to the completely compressed position of the sealingsurfaces, as shown in FIG. 8, the powder body sinks into the sealingsurface, the seat material being plastically deformed.

When the valve reopens, the particles are blown away by the outflow ofgas, as shown in FIG. 9, and at the same time the seat materials springback to their unloaded condition. To the extent a plastic deformationhas occurred of one or both seat surfaces, a permanent dent mark 11 willbe present in the sealing surface with a smaller depth than the largestindentation made by the powder body. The higher the yield strength ofthe seat material, the smaller the dent mark.

Examples of analyses for suitable materials will now be described. Allamounts are stated in per cent by weight, and inevitable impurities aredisregarded. It should also be mentioned that indications of yieldstrengths in the present description mean yield strengths at atemperature of approximately 20° C., unless another temperature isindicated. The alloys are chromium-containing nickel base alloys (ornickel-containing chromium base alloys). They have the property thatthere is no proper correlation between the hardness of the alloy and itsyield strength, but on the contrary probably a correlation betweenhardness and tensile strength. In connection with these alloys, theyield strength means the strength generated by a strain of 0.2(R_(p0.2)).

The alloy NIMONIC Alloy 105 has a nominal composition of 15% Cr, 20% Co,5% Mo, 4.7% Al, up to 1% Fe, 1.2% Ti and a balance of Ni.

The alloy NIMONIC 80A comprises up to 0.1% C, up to 1% Si, up to 0.2%Cu, up to 3% Fe, up to 1% Mn, 18-21% Cr, 1.8-2.7% Ti, 1.0-1.8% Al, up to2% Co, up to 0.3% Mo, up to 0.1% Zr, up to 0.008% B, up to 0.015% S anda balance of Ni.

The alloy NIMONIC 80 nominally comprises 0.04% C, 0.47% Si, 21% Cr,0.56% Mn, 2.45% Ti, 0.63% Al and a balance of Ni.

The alloy NIMONIC 81 comprises up to 0.1% C, 29-31% Cr, up to 0.5% Si,up to 0.2% Cu, up to 1% Fe, up to 0.5% Mn, 1.5-2% Ti, up to 2% Co, up to0.3% Mo, 0.7-1.5% Al and a balance of Ni.

The alloy NIMONIC PK50 nominally comprises 0.03% C, 19.5% Cr, 3% Ti,1.4% Al, up to 2% Fe, 13-15.5% Co, 4.2% Mo and a balance of Ni.

The alloy Rene 220 comprises 10-25% Cr, 5-25% Co, up to 10% Mo+W, up to11% Nb, up to 4% Ti, up to 3% Al, up to 0.3% C, 2-23% Ta, up to 1% Si,up to 0.015% S, up to 5% Fe, up to 3% Mn and a balance of Ni. Nominally,Rene 220 contains 0.02% C, 18% Cr, 3% Mo, 5% Nb, 1% Ti, 0.5% Al, 3% Taand a balance of nickel. Deformation combined with precipitationhardening can achieve an extremely high yield strength in this material.At a degree of deformation of 50% at 955° C., the yield strength becomesapproximately 1320 MPa; at a degree of deformation of 50% at 970° C.,the yield strength becomes approximately 1400 MPa; at a degree ofdeformation of 50% at 990° C., the yield strength becomes approximately1465 MPa, and at a degree of deformation of 25% at 970° C., the yieldstrength becomes approximately 1430 MPa. Precipitation hardening hasbeen applied for 8 hours at 760° C. followed by 24 hours at 730° C. and24 hours at 690° C.

Concerning the nominal analyses stated above it is obvious that inpractice, depending on the alloy actually produced, deviations maynaturally occur from the nominal analysis, just as inevitable impuritiesmay also occur for all analyses.

Technical literature describes in detail how to heat treat the variousalloys to generate precipitation hardening, and the heat treatment forsolution annealing and the recrystallisation temperatures of the alloysare also well-known.

The thermo-mechanical deformation process for increasing the yieldstrength involves a hot/cold-working of the material by well-knownmethods, for example, by means of rolling or forging of the seat area orotherwise, such as beating or hammering thereof. After deformation thesealing surface of the seat can be ground in.

To reduce the forces required at the thermo-mechanical deformationprocess the body with the seat area can before deformation be exposed tosolution annealing, for example for 0.1-2 hours at a temperaturenormally ranging between 1000 and 1200° C., depending on the analysis ofthe material, followed by quenching either in a salt bath to anintermediate temperature (typically 500° C.) followed by air cooling toroom temperature or quenching in gases to room temperature. Ahot/cold-working can then be carried out after these steps. To keep theforces suitably low the deformation preferably takes place at a raisedtemperature of about 900-1000° C., viz., below or around the lower limitfor the recrystallisation temperature, which is typically approximately950-1050° C. In this case with hot-working, a cooling from the solutionannealing to approximately the recrystallisation temperature canadvantageously be carried out without first cooling to room temperature.Possibly the deformation can be carried out in several steps withintermediate reheating. At a cold-working of approximately 20% it istypically possible to achieve a yield strength of 1200 MPa. If anespecially high yield strength is desired, after completed deformationand working the seat area can be exposed to precipitation hardeningwhich may, for example, take place for 24 hours at a temperature of 850°C. followed by 16 hours at a temperature of 700° C.

The base body treated as described above can be manufactured by means ofcasting and conventional forging or alternatively by means of a powdermetallurgical compacting process, such as a HIP process or a CIP processin combination with hot extrusion or a similar deformation process.

The shaft of the valve may be of a material different from that of thedisc and in that case can be friction-welded on to the disc.

What is claimed is:
 1. An exhaust valve for an internal combustionengine having a stationary valve member with a seat area, wherein theexhaust valve comprises a movable spindle with a valve disc having anupper surface with an annular seat area, said valve disk and seat areabeing of a nickel-based alloy, and wherein said seat area of saidnickel-based alloy has been given dent mark preventing properties in theform of a yield strength (R_(p0.2)) of at least 1000 MPa at roomtemperature by means of at least a thermo-mechanical deformation processinvolving coldworking.
 2. An exhaust valve according to claim 1, whereinsaid cold-working of the material has taken place at a temperature lowerthan the recrystallisation temperature of the alloy.
 3. An exhaust valveaccording to claim 1, wherein said cold-working of the material hastaken place at a temperature around the recrystallisation temperature ofthe alloy.
 4. An exhaust valve according to claim 1, wherein after thecold-working, the yield strength of the alloy has been further increasedby means of a precipitation-hardening heat treatment.
 5. An exhaustvalve according to claim 1, wherein the seat material has a yieldstrength of at least 1100 MPa.
 6. An exhaust valve according to claim 1,wherein the seat material has a yield strength of at least 1200 MPa. 7.An exhaust valve according to claim 1, wherein the seat material has ayield strength of at least 1300 MPa.
 8. An exhaust valve according toclaim 1, wherein the seat material has a yield strength of at least 1400MPa.
 9. An exhaust valve according to claim 1, wherein the seat areas onthe stationary member and the valve disc, respectively, have mainly thesame yield strength at operating temperatures of the seat areas.
 10. Anexhaust valve according to claim 1, wherein the seat area on the valvedisc has a substantially lower yield strength than the seat area on thestationary member at operating temperatures of the seat areas.
 11. Anexhaust valve according to claim 1, wherein said valve disc has anexternal diameter in the range from 130 mm to 500 mm.
 12. An exhaustvalve according to claim 11, wherein said exhaust valve is a two-strokecrosshead engine exhaust valve.
 13. An exhaust valve for an internalcombustion engine having a stationary valve member with a seat area,wherein the exhaust valve comprises a movable spindle with a valve dischaving an upper surface with an annular seat area, said valve disk andseat area being of a nickel-based olloy having a recrystallisationtemperature, and wherein at least said seat area has been manufacturedby means of either casting or powder metallurgical application followedby thermo-mechanical deformation at a temperature lower than or aroundthe recrystallisation temperature of the alloy and with a degree ofdeformation providing the seat area with dent mark preventing propertiesin the form of a yield strength (R_(p02)) of at least 1000 MPa at roomtemperature.
 14. An exhaust valve according to claim 13, wherein theseat material has a yield strength of at least 1100 MPa.
 15. An exhaustvalve according to claim 13, wherein the seat material has a yieldstrength of at least 1200 MPa.
 16. An exhaust valve according to claim13, wherein the seat material has a yield strength of at least 1300 MPa.17. An exhaust valve according to claim 13, wherein the seat materialhas a yield strength of at least 1400 MPa.
 18. An exhaust valveaccording to claim 13, wherein said valve disc has an external diameterin the range from 130 mm to 500 mm.
 19. An exhaust valve according toclaim 18, wherein said exhaust valve is a two-stroke crosshead engineexhaust valve.
 20. A new use for a nickel-based chromium-containingalloy as a dent mark limiting material in a valve disc seat area,wherein an exhaust valve for an internal combustion engine has a movablevalve disc with an annular seat area, said valve disk and said seat areabeing provided with a nickel-based alloy having a yield strength of atleast 1000 Mpa at room temperature.